These are very tricky to fit! Remove needle and pull the threader mechanism carefully off the shaft so the parts remain intact. Carefully replace just this part and re-assemble pushing onto shaft and locating the positioning pin in groove.

Watch a video on replacing a similar type of threader onto a QC1000 machine. Please be aware that each machine varies slightly and this video is a guide. The 3000 series threader doesn't need to be located in the pin guide after pushing onto the shaft like the machine in this video.